What is this gauge used for?

[splat]

[splat]

It has a zero/reset button behind the display.
It measures in thousandths of an inch.
So far, it's a rubic's cube to me.

[splat]

It appears to be missing the trailer hitch ball....
I only bought it cause someone said it could measure afterbang.



Actually, it was given to me by someone who had no idea what it was and thought I could use it.